Cook the bacon in a large skillet over medium heat until crisp. Transfer to a paper towel-lined plate to drain, then crumble once cooled.
Wash, peel, and quarter the potatoes. Place them in a large pot and cover with cold water.
Bring the potatoes to a boil over high heat. Reduce the heat and simmer for 10–15 minutes, or until they are fork-tender.
Drain the potatoes well and return them to the warm pot.
Add the butter, milk, and salt. Mash with a potato masher or use a hand mixer until smooth and fluffy.
Gently stir in the sour cream and 1 cup of the shredded cheddar cheese until well combined.
Transfer the mashed potatoes to a greased 9x9-inch baking dish or 2-quart casserole dish.
Top with the remaining 1 cup of cheddar cheese and the crumbled bacon.
Cover loosely with foil and bake for 10–15 minutes.
Remove the foil and continue baking for 10 minutes, or until the cheese is melted and bubbly.
Sprinkle with the chopped chives just before serving. Serve warm.
Notes
Any starchy potatoes will work for this recipe. Make sure to shred your own cheese instead of using pre-shredded for the best results. This casserole can easily be made the day before serving.Assemble and wait to top with the shredded cheese and bacon before baking and serving. Refrigerate any leftovers in an airtight container for up to 3-4 days.